Signal ratio measuring system providing constant servo loop gain



Feb. 9, 1965 A0. HOLDO 3,159,220

SIGNAL RATIO MEASURING SYSTEM PROVIDING CONSTANT SERVO LOOP GAIN Filed D60- 21, 1959 2 Sheets-Sheet 1 QIAT\O4 iq-E a a m a) M 25$ q-QJ-D ANDRES 0. HOLDO INVENTOR.

A 77ORNEYS Feb. 9, 1965 A. o. HOLDO I 3,1695220 SIGNAL RATIO MEASURING SYSTEM PROVIDING Y cousmm' SERVO LOOP GAIN Filed Dec. 21, 1959 2 Sheets-Sheet 2 ANDRE-S 0. H0400 INVENT OR.

United States Patent h 3,169,220 SIGNAL RATIO MEASURING SYSTEM PROVIDING CONSTANT'SERVO LOOP GAIN Andres O. 'Holdo, Pasadena, Calif., assignor to F. L. Moseley Co., a corporation of California Filed Dec. 21, 1959, Ser. No. 860,774

3 Claims. (Cl. 324.140)

This invention relates to electrical signal measuring systems and more particularly to a new and improved system for measuring the relative magnitudes of 'two electrical signals. Y

Patented Feb. 9., 1965 of the applied input voltages. An indicator is linked to the servo system from which the ratio between the applied electrical signalsmay be directly measured.

p In the testing of electrical circuit components and in the monitoring of electrical signals, it is frequently desirable to derive anoutput indication of the ratio of the magnitudes of two separate signals. For example, in

the measurement of transistor characteristics, it is fre-' quently desirable to derive a signal representing beta, which may be definedas the ratioof the collector current to the base current. Similarly, the gain of an amplifier is defined as the ratio of the valueof an output signal to the value of an input signal, and again it is'desirable to provide a means for directly generating a signal corresponding to the ratio withoutth'e necessity for manual computation. 1

One problem encountered in systems'employing closed servo loops, is that the overall gain of the servo loop may varyinaccordance with thevalue of a signalapplied to the input circuit so that the overall loop gain becomes a-function of the value of the applied signal. The present invention is directed to an arrangement for solving the problem set forthabove and may be employed for the purpose of measuring the ratio between the magnitudes of'two electrical signals as'well; as for controlling the gain of an element in a closed loop'ser'vo systemin a predetermined fashion corresponding to the magnitude of an appliedsignal. I q I Accordingly, it is a primary object ofthepresentjinvention to provide a newland improved ratio measuring servo system;

, It is another objectof the present invention to provide vention;

A better understanding of the present invention may be had from a reading of the following detailed description and an; inspection of the drawings, inwhichz.

FIG. 1 is a simplified block and schematic diagram of a ratio measuring servo system in accordance withthe inlfIG, 2.is a schematic circuit diagram of one form of a ratio measuring system in accordance with the invention for performing thefunctions indicated by the diagrammatic illustration of FIG. 1. l In FIG. 1 there is illustrated a ratio measuring system n accordance with the invention in which a signal or voltage e may be applied between a pair of input terminals 1 and 3 and a second signal or voltage e may be apbalance, the potentiometer plied between the input terminals 3 and .5. The voltage e 'appears across a balancing element in the, form of a potentiometer 6 having a movable contactor which is connected to a servo amplifier 7. Between the contactor and the terminal 1 there appears a voltage e, termed the error voltage, which is appied to the input stages 7A of theservo amplifier 7 and reflects a condition of balance or unbalance within the input circuit. Ina conditionof v contactor is adjusted until the voltage e is substantially equal and opposite to a fractional part of the voltage e appearing between the contactor and the 'lefthand end of the potentiometer resistance element. i

The position of the contactor of the potentiometer 6 1s measuredas afunction of its position and is designated by the symbol 06. When the input circuit is in a condition of balance, the error signal e is substantially equal to zero and the servo amplifier 7 provides a substantially zero output signal. However, in a condition of unbalance, the error signale is passed and amplified by the servo amplifier input stages 7A to a gain control network via 'a coupling caa new andimprovedservo system in which the gain of the system is varied in a" predetermined functional relationship with respect to an applied Tsignali It is yet anotherobject of the present invention to provide a ,new and improved ratio meter. ItQisstillQa further object offthe present invention to provide, a new ,and'improved system for deriving an output indication directly representing the ratio of the magnitudes oftwo applied signals; a

BriefiyQin accordance. with one aspect of the invention, a system is provided for measuring the relative magnitudes of'two input signalsin which a closed loop servo system maintains a condition of balance inan input 'circuit with the" gain of an element contained within the' 'servoloop being altered to maintain a predetermined overall servo loop gain substantially unaffected bythemagnitude of the applied input signals. In a particular embodiment of the invention, a servo loop is employed in Whichaservo am: plifier is connected to again control circuit within which the gain of the servo'a'mplifier isaltered as a function of an applied signal to maintain the overall servo loop gain constant. g .1 5

.In accordance with another aspect of ,the present in: vention, a new and improvedratio meter is provided in the servo amplifier, the

which an inputcircuitreceives; two separate electrical signals which are balanced against oneanother. In a condition of unbalance, an error, signal is provided which is amplified by a. servo amplifier whose gain{ is determined as in inverse Qfunction of one ofthe signals. Theiuput circuit isfrebalanced as a function of the output-signal from the servo amplifier with an overall servo loop being pacitor Q The gain control network comprises a fixed lmpedance'in the form of -aresistor 9 which cooperates With'a pair of serially connected diodes 10 and 11 to,

variablyattenuate the output signal from the servo amplifier input stages 7A. 1 i a Between the input stages 7A' and output stages 7Bof gaincontrol network comprising the fixed impedance 9 and the "variable impedances provided the diodes 10 and 11 is connected. The diodes l0 and 11 are connected serially with a common junctron point 14 being connected to receive the signal passed by the resistor 9. Furthermoregthesignal e applied to the terminals 3 and 5 is applied to the diodes 10 and 11 via a fixed'resistor' 12. A capacitor 13 functions to pro vide a low impedance pathto ground for amplified signals passing through the diode .10. 1

flhe current fiowthrough the diodes ltltand ll is a funct on of the voltage je andfthe impedance 'of'the diodes" varies as a function of'jthecurrent new therethrough so that the impedance from the junctionpointldto greens; 1

By this means, a

The servo amplifier output stage 7B provides an output signal which is applied to one winding of a reversible two-phase motor 15. In accordance with common practice, one winding oi -the two-phase motor 15 receives an alternating current waveof fixed'phase, while theservo amplifier output stage 7B} provides an output signal of l reversible phase and variable amplitude corresponding .to

the polarityand magnitude of the error voltage e,applied,

to the servo amplifier input stages 7A. In response to the signal from the servo amplifier output stages 7B, the reversible phase motor 15 turns in a direction determined by the phase relationship between the output signal and the fixed phase wave applied to the second winding.

The motor 15 may be mechanically linked to the potentiometer 6 by means of a suitable mechanical linkage indicated diagrammatically in FIG. 1 by means of the dashed line 14'. As a result, the motor 15 rotates in response to the error signal e to move the contactor of the potentiometer 6 in a direction which reduces the error voltage e and restores a condition of balance within the input circuit. In a condition of balance, the position, on, of the contactor of the potentiometer 6 corresponds to the ratio of the voltage 2 to the voltage e This ratio may be read directly in the arrangement of FIG. 1 by means of a suitable indicating scale 17 which is mechanically linked to the motor 15 and contactor of the potentiometer 6 by the mechanical linkage 16.

The manner in which the above system operates to provide an accurate measurement of the ratio of the magnitudes of applied input signals may be best understood by a consideration of the following relationships:

where e equals the error signal, e equals one applied input signal, e equals the other applied input signal, and a equals the position of the contactor of the potentiometer 6 given asa percentage of its travel along the length of the resistance element of the potentiometer 6. The above relationship obtains so long as a condition of balance exists in which the error signal e is substantially equal to zero. Therefore,

The loop gain for a servo system with constant ampliez 1 V011. the current, I, through the diodes of FIG. 1 is PR2 v where R equals the value of the resistor 12. This relationship obtains since the impedance of and 11 is relatively small compared to R The equation for the impedance of one of the diodes is 1=n 1 where I equals the saturation current in the reverse direction of the diode. Therefore,

If the current I is much greater than the saturation current I q I Since K' is a constant, it is established that the impedance Z is an inverse function of the current I passed by the resistor 12. Furthermore, since the current I through the resistor 12 bears a direct relationship to the applied voltage e the impedance is an inverse function of the magnitude of the voltage e The result is that a variable impedance is presented to the signal passed by the resistor 9 in the arrangement of FIG. 1 as an inverse function of the applied voltage e so as to alter the gain of the servo amplifier 7 and to establish a constant servo loop gain. Therefore, the arrangement of FIG. 1 functions as desired to measure theratio between the voltages e the diodes 10 and e irrespective of the magnitude of the voltages so long as the diodes ltl and 11 are maintained in a conductive condition under the influence of the applied voltage e FIG. 2 is a schematic circuit diagram of a portion of an arrangement corresponding to FIG. 1 illustrating the details of one suitable servo amplifier and gain control circuit arrangement for use in a ratio measuring instrument. In FIG. 2, similar reference characters have been employed to designate each of the component parts corresponding to similar parts shown in FIG. 1 and described above. Accordingly, the input signal voltages e; and e are applied respectively between the terminals 1 and 3 and the terminals 3 and 5 with a potentiometer 6 being adapted to establish a condition of balance within the input circuit so that the voltage e substantially disappears in a condition of balance and appears only upon a change in one of the voltages e or e causing an un balanced condition.

With constant voltages applied to the terminals 1, 3 and 5, and the potentiometer 6 in a proper position, the systemis at rest and the input circuit may be said to be in abalancedcondition. However, upon a variation of either of the voltages e or 2 an error voltage e is provided which is applied to the moving contact of a suitable modulator which in the arrangement of FIG; 2 takes the formof a double pole single throw vibratory switch 20 which maybe energized by a coil 21, The center tap of the primary winding 22 of an input transformer 23 is connected to the movable contactor of the potentiometer 6 so that the modulator 20 functions to alternatively andoppositely apply the error voltage e to the ends of the primary winding 22. The result is that an alternating current wave appears across the secondary winding 24 having an amplitude and phase corresponding to the magnitude and polarity of the error voltage e.

A smoothing capacitor 25 functions to provide a substantially sine wave alternating current for application to the control electrode of a conventional amplifier electron tube 26. In well known fashion, the control electrode of the electron tube 26 is returned to ground via a grid resistor 2'7, the cathode is connected to a cathode resistor 28 and bypass condenser 29, and the anodeof the electron tube receives an operating voltage via a load resistor 30 from a power supply terminal 31. A decoupling capacitor 32 connected between the anode. of the electron tube 26 andground reference potential bypasses unwanted high frequency components to ground, and the amplified alternating current wave is passed to the grid of a second electron tube 33 via a coupling capacitor 34 and a pair of resistors 35 and 36 which function as a voltage divider. The amplifying electron tube 33 receives its operating potential via a load resistor 39 and is self-biased by means of the cathode resistor 37 and bypass capacitor 38.

The output signal from the anode of the amplifier tube 33 ispassed to a gain control network via a coupling capacitor 8 corresponding to that illustrated in FIG. 1. A fixed impedance in the form of a resistor 9 is connected ing therethrough.

As shown in FIG. 2, a bias voltage may be applied to the diodes and 11 from a suitable source 40 by means of an adjustable resistor 41 to insure that the diodes 10 and 11 operate in a preselected region of their response characteristics. For example, by applying a suitable bias voltage to the diodes 10 and ill, a linear relationship between applied voltage and impedance may be obtained.

As in FIG. 1, the voltage e is applied to the series connected diodes 10 and 11 via a resistor 12 so that in accordance with the above described considerations, the impedance presented to the signal passed by the resistor 9 varies as an inverse function of the applied voltage 2 so as to alter the gain of the servo amplifier to maintain the overall loop gain of the servo system constant.

The variably attenuated signal appearing at. the junction point 14 may be passed to a third amplifier stage 42 via a coupling capacitor 43. As before, the amplifier stage 42 may receive operating voltage via a load resistor $4 from the terminal 31, the control electrode may be returned to ground via a grid resistor 45, and the cathode may be connected to a cathode resistor 46 which is bypassed by a suitable capacitor 47. The amplified output signal from the electron tube'42 may be passed by acapacitor 48 to successively connected amplifier stages similar to those previously described. In FIG. 2 for convenience, additional amplifier stages have been omitted from the drawing with the connection to the output stages beingbroken to indicate that additional amplifier stages may beemployed if desired. In any event, the amplified output signal appears across a resistor 49 connected between the control grids of a pair of outputelectron tubes 50 and 51.

The output tubes 50 and 51 function as power amplifiers to drive a primary winding 52 of an output transformer 53. The screen electrodes of the output tubes 50 and 51 are held at fixed potential by means of a resistor 57 with the anodes of the output tubes 50 and 51 receiving operating potential via the center tap of the primary winding 52.

. terminal 31. It will be understood that eachof the elecbalancingmeans for establishing a condition of balance A capacitor 58 may be connected across the primary winding 52 to form a resonant circuit at the frequency of the wave generated by the actionof the vibratory switch 20.

A secondary winding 54 of the transformer 53 is connected directly to one winding 55 of the two-phase revers- 1 ible motor 15. As described above in connection with FIG. 1, another winding 56 of the motor 15 receives a wave of reference phase which bears a fixed phase relationship to the wave applied to the energizing coil 21 associated with the vibratory switch 20. The wave applied to the motor winding 55 functions to cause the motor 15 to rotate in a selected direction and to an extent dependent upon the amplitude and phase of the amplified alter nating current Wave. The motor 15 is linked to the potentiometer 6 via a mechanical linkage 16 so that in response to the error signal e, the arrangement of FIG. 2 functions to cause the motor'15 to rotate in a direction which brings the input circuit into a condition of balance in which the position of the contactor on the potentiometer 6 corresponds to the ratio between the voltages e and e As a result, the mechanical linkage ldmay be extended to position an indicator or to control auxiliary tron tubes contains, a heater for elevating the temperature of the cathode to electron emissive temperature which is connected to a suitable power supply source, although the symbols and connectionsfor such heaters have been omitted from FIGJZ for convenience of illustration.

Potentiometere 5,000 ohms. Resistor 9 150,000 ohms. Resistor 12 47,000 ohms. Resistors 2 7', 35, 45 1 megohm.

Resistors 20, 37, 4-6, 57 4,700 ohms. Resistors 30, 39, 44-, 49 220,000 ohms.

Althouglrthere has been described above a specific arrangement of the invention for measuring the relative magnitudes of two separate applied voltages, it will be appreciated that the invention is not limited to the specific arrangements shown. Accordingly, the invention should be considered to include any variations, modifications or alternativestructuresfalling within the scope of the annexed claims.

What is claimed is:

1. In 'a system for measuring the-ratio between the magnitudes of first and second electrical signals thecombination comprising an input circuit for receiving said electrical, signals, said input circuit including adjustable between said first and second electrical signals, a servo amplifier coupled to said input circuit for receiving an electrical signal from said input circuit when a condition of unbalance exists between said first and second electrical signals, variable attenuator means whose impedance is a function of the current therethrough coupled to said servo amplifier to control the gain thereof, means energizing said variable attenuator means with a current whose magnitude varies in accordance with the variations in magnitude ofsaid second electrical signal to vary the gain'of the servo amplifier as a function of the second electrical signal and independently of any variations in magnitude of said first electrical signal, servo positioning means coupling said servo amplifier to said adjustable balancing means for establishing a condition of balance between said first and second electrical signals, and means coupled to said adjustable balancing means for indicating ance and a pair of serially connected diodes whose conductance is varied as a function of said second electrical signal.

3. The combination according to claim 1 comprising biasing means coupled to said variable attenuator means for establishing a particular attenuation range so as to determine the particular functional relationship between servo amplifier gain and applied signal magnitude.

References Cited in the file of, this patent UNITED STATES PATENTS 2,543,469 .Roston Feb. 27, 1951 2,547,703' Hermont Apr. 3, 1951 2,715,209 Williams Aug.'9, 1955 2,790,146 Livingston Apr. 23, 1957 2,913,668 ,Lide Nov. 17,: 1959 Dodge June 28 1960 

1. IN A SYSTEM FOR MEASURING THE RATIO BETWEEN THE MAGNITUDES OF FIRST AND SECOND ELECTRICAL SIGNALS THE COMBINATION COMPRISING AN INPUT CIRCUIT FOR RECEIVING SAID ELECTRICAL SIGNALS, SAID INPUT CIRCUIT INCLUDING ADJUSTABLE BALANCING MEANS FOR ESTABLISHING A CONDITION OF BALANCE BETWEEN SAID FIRST AND SECOND ELECTRICAL SIGNALS, A SERVO AMPLIFIER COUPLED TO SAID INPUT CIRCUIT FOR RECEIVING AN ELECTRICAL SIGNAL FROM SAID INPUT CIRCUIT WHEN A CONDITION OF UNBALANCE EXISTS BETWEEN SAID FIRST AND SECOND ELECTRICAL SIGNALS, VARIABLE ATTENUATOR MEANS WHOSE IMPEDANCE IS A FUNCTION OF THE CURRENT THERETHROUGH COUPLED TO SAID SERVO AMPLIFIER TO CONTROL THE GAIN THEREOF, MEANS ENERGIZING SAID VARIABLE ATTENUATOR MEANS WITH A CURRENT WHOSE MAGNITUDE VARIES IN ACCORDANCE WITH THE VARIATIONS IN MAGNITUDE OF SAID SECOND ELECTRICAL SIGNAL TO VARY THE GAIN OF THE SERVO AMPLIFIER AS A FUNCTION OF THE SECOND ELECTRICAL SIGNAL AND INDEPENDENTLY OF ANY VARIATIONS IN MAGNITUDE OF SAID FIRST ELECTRICAL SIGNAL, SERVO POSITONING MEANS COUPLING SAID SERVO AMPLIFIER TO SAID ADJUSTABLE BALANCING MEANS FOR ESTABLISHING A CONDITION OF BALANCE BETWEEN SAID FIRST AND SECOND ELECTRICAL SIGNALS, AND MEANS COUPLED TO SAID ADJUSTABLE BALANCING MEANS FOR INDICATING THE RATIO BETWEEN SAID FIRST AND SECOND ELECTRICAL SIGNALS. 